In seismic exploration, generated acoustic waves are transmitted downward into the earth. At the transition zones between various types of rock strata, reflections of the acoustic waves occur, and attenuated parts of the reflected waves get back to the earth's surface where they are detected by groups of geophones planted at the earth's surface. Each geophone group (which can comprise a number of individual geophones) produces an electrical signal in response to the detected acoustic wave, and each geophone group transmits its electrical signal to a recording truck nearby.
Seismic detecting equipment, beginning with the primary detectors, the geophones (or hydrophones, in the case of marine exploration), must be tested occasionally to make certain that the rough field use it customarily receives has not impaired its sensitivity and general usefulness. To date, there are no standard tests for geophones, but certain measurable properties have begun to be recognized as diagnostic, e.g., the resonance frequency, the impedance, and the damping factor. (The following references go into detail about previous measurements of these properties: Mark Products, Inc., Catalogue, February 1968, pp. 3-14; "The Phone" published by Walker-Hall-Sears, Inc., Spring 1974; and U.S. Pat. No. 3,717,810 to F. I. Spanbauer, Feb. 20, 1973). The present invention is concerned mainly with the measurement of one of these mentioned properties, the resonance frequency.